Integration of the retrovirus genome into the chromosomes of a host cell is an essential step in the propagation of this class of virus. Although restriction endonuclease analysis has indicated that this integration is random, our data using in situ hybridization has shown a very nonrandom pattern of endogenous retrovirus loci in White Leghorn chickens. This pattern can be accounted for only by specific integration sites or by transposition. Current research is centered on understanding the mechanism involved in this nonrandom distribution and to access how widespread it is in different avian breeds. Analysis of exogenous retrovirus integration will also be analyzed by in situ hybridization and the results compared with restriction endonuclease analysis. Finally, any preferred integration sites of oxogenous retroviruses will be compared with the location of known cell oncogenes.